From: Brian Candler <b.candler@...>
Date: 2009-01-13T01:04:55+09:00
Subject: Re: functional programming

Mike Gold wrote:
> Brian Candler wrote:
>> (2) to understand Mike's assertion 
>> that Ruby lambdas are not first-class functions (i.e. "The difference 
>> between lambda { } and a real first-class function is quite profound").
> 
> Ruby:
> 
>   mean = lambda { |*args|
>     (args.inject(0) { |acc, x| acc + x })/args.size.to_f
>   }
>   data = [[1,2,3], [4,5,6,7]]
> 
>   data.map(&mean) # fail
>   data.map(&:mean) # fail
>   data.map { |*numbers| mean.call(*numbers) } # fail
>   data.map { |numbers| mean.call(*numbers) } # phew, found it

OK, I think I see what you're getting at. You've written a lambda with a 
splat (i.e. which takes a variable number of arguments), but needed a 
splat at the caller side too. Otherwise the lambda receives a single 
argument, which is an Array object.

You could have written the lambda to take a single Array argument:

  mean = lambda { |arg|  arg.inject(0) ...etc }

I don't know enough about Lisp to say how Lisp resolves the same 
dichotomy, but surely it must exist. That is, there must be a difference 
between calling a function foo with a (list of) n arguments, and calling 
a function foo with 1 argument which is itself a list?
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